U.S. intelligence agencies and the Federal Bureau of Investigation have spent months trying to substantiate explosive claims, compiled by a former Western intelligence official, that Russian government operatives engaged in an extensive conspiracy with advisers to Donald Trump’s presidential campaign and employees of his company, people familiar with the matter said.

The unverified allegations—including a claim Russia has material that could be used to blackmail Mr. Trump—were deemed sufficiently significant by senior intelligence officials to summarize them in a two-page addendum to the classified briefing President-elect Trump received last Friday about Russian efforts to influence the U.S. presidential campaign, the people said.

U.S. officials confirmed that a summary of the information had been given to Mr. Trump. They said sharing of such unverified information was taken out of an abundance of caution that the incoming president should be aware of allegations being made against him that could become public—a decision intelligence experts backed. President Barack Obama received the same information, officials said.

The agencies are continuing to investigate the claims, the people familiar with the matter said.